We need to keep track of both wall time which are used for pauses
and actual animation driver time which is used for actual animations.
When switching between these, we need to also maintain the temporal
drift potentially introduced by the driver and also the time
that has passed in wall-time from when a pause has started until
an action animation takes over.

This change introduces a well defined elapsed() function in
QUnifiedTimer which will return the right value based on which
mode we are currently in. It also introduces start/stopAnimationDriver
functions which helps us maintain the temporal drift and pause-delta.